You need to file an amended return for 2017 to carry any credit back.
According to the IRS:
"If you can't use part or all of your general business credit because of the tax liability limit (Part II, line 38, is less than the sum of Part I, line 6, and Part II, lines 25 and 36), carry the unused credit back 1 year. If the credit is for oil and gas production from marginal wells, the carryback period is 5 years. To carry back an unused credit, file an amended return (Form 1040X, Amended U.S. Individual Income Tax Return, Form 1120X, Amended U.S. Corporation Income Tax Return, or other amended return) for the prior tax year or an application for tentative refund (Form 1045, Application for Tentative Refund, or Form 1139, Corporation Application for Tentative Refund). Generally, if you file an application for a tentative refund, it must be filed by the end of the tax year following the tax year in which the credit arose."
https://www.irs.gov/instructions/i3800
